1. Field of the Invention
The present invention relates to a method for producing a semiconductor device such as a memory, a photoelectric converting device, a signal processing device or the like adapted for use in various electronic appliances, and more particularly to a method for producing a semiconductor device featuring an alignment mark structure.
2. Related Background Art
In the formation of wiring layers by patterning in conventional semiconductor devices, the precision of the patterning is improved by automatic alignment utilizing a recess, formed in a predetermined position in advance, as a mark. For example, in a conventional semiconductor device including a CMOS transistor as shown in FIG. 1, oxide films 2, 4 on the principal surface of a substrate 1 are partially removed at a predetermined position by selective etching to expose the silicon surface, thus forming a mother mark 6. Then, the entire substrate surface, including the mother mark 6, is covered by a metal film 9, thereby forming a recess 10 on the metal film, corresponding to the form of the mother mark 6. A wiring layer of a desired pattern is obtained by patterning the metal film, utilizing auto-alignment achieved by irradiating the recess with a laser beam and detecting the reflected light, or by utilizing data obtained by image processing such as TV-AA (television auto-alignment).
However, in such a conventional semiconductor device, if the metal film is formed by a selective metal depositing method such as tungsten chemical vapor deposition method (CVD) to a thickness required for the wiring layer, the mother mark area and the metal film therearound become flat so that the recess corresponding to the mother mark is no longer formed, because of the selective growth characteristic of the depositing method. Consequently the detection of the mother mark, essential for auto alignment, becomes difficult and the precision of alignment significantly deteriorates.
FIG. 2 shows the wiring layer formed by selected deposition utilizing the tungsten CVD mentioned above.
On the mother mark 6, a metal film composed of tungsten is deposited to the level of the insulation film 4, so that the portion 10, which is to serve as an alignment mark in a next step, has become flat.